Transmit and receive antennas are often close to each other on ships, satellites, and spacecraft, for example. Precautions must therefore be taken to prevent the transmitted signals interfering with the receive channel. The transmit frequency bands are generally separate from the receive frequency bands. However, this separation is not always sufficient because interfering signals often occur outside the transmit band.
The interfering signals include signals resulting from the combination of signals from two or more transmissions on different carrier frequencies. As the combination is algebraic, it produces interfering signals at frequencies in the transmit bands and in the receive bands.
Accordingly, when two signals centred on frequencies F.sub.1 and F.sub.2 pass through the same amplifier, unwanted signals are generated centred on the frequencies F.sub.1 -F.sub.2, F.sub.1 +F.sub.2, 2F.sub.1 +F.sub.2, etc.
The interfering signals can also comprise harmonics of a monotone (single frequency) signal.
The interfering signals originate in non-linearities of the transmission system, in particular in the amplifiers. The fact that it is difficult to decouple the transmit antennas from the receive antennas makes them more of a problem.
Various solutions have been proposed to the problem of eliminating intermodulation signals in multitone (i.e. multi-frequency) broadband transmissions. Those solutions usually consist in modifying the signals to be transmitted to correct the non-linear element.
Prior art methods do not provide a satisfactory solution.